Discovered while poking around in #3612177: Update autoload classmap for Symfony runtime and etc.
PageCache::set() stores a complete HtmlResponse object.
HtmlResponseAttachmentsProcessor leaves the attachments on the response, so every cache hit unserializes the library list, the drupalSettings array, the html_head render arrays and the CacheableMetadata.
PageCache::lookup() only uses the status, headers and content.
Only cache a Response containing the data that PageCache::lookup() needs.
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Comment #2
longwaveImplemented the change and a test.
In the test this saves about 1.7Kb per page cache entry, but this is in the Stark theme - realistic themes will have more libraries, html_head entries, etc.